Aromatics complexes generally derive their feed from a catalytic reformer, although other sources of mixed xylenes feeds are possible, such as those derived from pyrolysis gasoline from olefin crackers. The reformate product from the reformer contains benzene (Bz), toluene (TOL), C8 aromatics (ethylbenzene (EB), and the three xylene isomers of paraxylene (pX), metaxylene (mX), and orthoxylene (oX)), and C9+ aromatics that are primarily C9 and to a lesser extent C10+ aromatics. Most aromatics complexes focus on the production of paraxylene and benzene, and occasionally orthoxylene and metaxylene, although the markets for orthoxylene and metaxylene are not as large as for paraxylene. Paraxylene is oxidized to terephthalic acid which is purified and polymerized with ethylene glycol to make polyester. Polyester is used in making clothing, film and bottles. Benzene is used in making many useful derivatives with end products such as polystyrene, nylon, polycarbonate, and phenolic resins.
To make the most amount of paraxylene and benzene from a given amount of reformate, aromatics complexes may comprise units that will convert the toluene and/or C9+ aromatics in the reformate to xylenes and benzene, including TOL/A9+ transalkylation (TOL/A9+ TA) units.
TOL/A9+ transalkylation usually takes place in the presence of hydrogen. Processing C9+ aromatics is complex, because there are a number of A9 and A10 isomers that can undergo a number of different reactions depending on the choice of catalyst. Reactions occurring in a TOL/A9+ transalkylation (TA) reactor include but are not limited to:TOL+TOL=Bz+XYL  (1)MEB+H2→TOL+C2  (2)TOL+TMB=2XYL  (3)C3Bz+H2→Bz+C3  (4)DMEB+H2→XYL+C2  (5)DEB+H2→EB+C2  (6)EB+C2→Bz+C2  (7)C4Bz+H2→Bz+C4  (8)C3XYL+H2→XYL+C3  (9)TMB+TOL=TTMB+Bz  (10)Where:                XYL=xylene isomers        MEB=methylethylbenzenes (3 isomers)        TMB=trimethylbenzenes (three isomers)        C3Bz=propylbenzenes (iso-propylbenzene=cumene and n-propylbenzene)        H2=hydrogen        C2=ethane        C3=propane        DMEB=dimethylethylbenzenes (6 isomers)        DEB=diethylbenzenes (3 isomers)        C4Bz=butylbenzenes (4 isomers)        C3XYL=propylxylenes (includes n-proylxylenes and iso-propylxylenes known as cymenes)        TTMB=tetramethylbenzenes (3 isomers)        
Note that MEB hydrocracking (Reaction 2) produces TOL, which can react with TMB to form xylenes (Reaction 3) or disproportionate to form xylenes and benzene (Reaction 1). Thus, A9+ can be a feed to a TOL/A9+ reactor alone, i.e., TOL in the feed can be zero.
MEB and propylbenzene hydrocracking (Reactions 2 and 4) can be driven to very high conversion. However, transalkylation reactions that produce xylene isomers (such as Reactions 1 and 3) are equilibrium limited. Thus, the reactor effluent from TOL/A9+ TA will contain light ends, including C2 and C3, Bz, XYL, and unreacted TOL and A9+. The reactor effluent is separated into streams rich in light ends, Bz, XYL, and unreacted TOL and A9+. The TOL and components of the A9+ are recycled to the reactor. Separation of the reactor effluent into these streams is energy intensive and represents a substantial portion of the variable cost for these processes.
Accordingly, there remains the need for other energy and capital efficient schemes for separating the reactor effluent from TOL/A9+ transalkylation reactors.